1. Field of the Invention
One embodiment of the invention disclosed herein relates to a contactless power feeding system.
2. Description of the Related Art
Various electronic devices have spread and a wide variety of products are shipped to the market. In recent years, portable electronic devices such as mobile phones and digital video cameras have widely spread. Further, electric propulsion vehicles that are powered by electric power, such as electric cars, are appearing on the market as products.
In such mobile phones, digital video cameras or electric propulsion vehicles, power storage devices serving as power storage means (referred to as batteries and storage batteries) are incorporated. Currently, such a power storage device is charged in most cases by being directly connected to a household AC power source which is a power feeding means. In addition, a unit without a power storage device or a unit which does not use electric power charged in a power storage device is directly supplied with power from a household AC power source through a wiring or the like so that the unit operates.
On the other hand, methods by which power storage devices are charged without making contact or power is supplied to loads without making contact have been researched and developed. Typical methods are an electromagnetic coupling method (also referred to as electromagnetic induction method, see Reference 1), a radio wave method (also referred to as microwave method), and a resonance method (also referred to as resonant method, see References 2 to 4).
As described in References 2 to 4, in a contactless power feeding technique using a resonance method, a device that receives power (hereinafter, referred to as power receiving device) and a device that feeds power (hereinafter, referred to as power transmitting device) each have a resonant coil. Further, in each of the power receiving device and the power transmitting device, an electromagnetic coupling coil is provided. Feeding power from a power source to the resonant coil in the power transmitting device and feeding power from the resonant coil to a load in the power receiving device are conducted by the electromagnetic coupling coils.
The resonant coil of the power transmitting device and the resonant coil of the power receiving device are adjusted to resonate (LC resonance) at the same frequency.
When the resonant coil of the power transmitting device and the resonant coil of the power receiving device face each other, they makes a resonant (resonance) phenomenon, and thereby efficient power transfer is realized even when the distance between the resonant coils are large (see Reference 6).
In recent years, identification technologies in which ID (identification number) is assigned to an individual object to clarify the history of information thereof, which is utilized for production, management, or the like have attracted attention. In particular, RFID (radio frequency identification) technology for transmitting and receiving data by wireless communication has been widely used (see Reference 5). A wireless communication system using RFID technology is a communication system including a wireless communication device (interrogator) and a data carrier (transponder), and data is wirelessly communicated between them. The wireless communication device refers to a device capable of wirelessly transmitting and receiving signals, such as a reader/writer, a mobile phone, or a personal computer. In this specification, the wireless communication device is typically referred to as a reader/writer. In addition, the data carrier is generally called an RFID, an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like. In this specification, the data carrier is typically referred to as an RFID.
Examples of RFIDs include passive RFIDs which do not have power sources and active RFIDs which incorporate power sources. The passive RFID has a function of converting a wireless signal (carrier wave or amplitude-modulated wave generated by superimposing modulation wave on carrier wave) transmitted from a reader/writer into a DC voltage by a rectifier circuit which is provided in the RFID, and circuits in the RFID operate using the DC voltage.